Abstract: Excessive application of phosphate fertilizers in agriculture leads to the loss of unused phosphorus, which poses a serious threat to water quality. Therefore, identifying key source areas of phosphorus loss in agricultural fields as well as their influencing factors is crucial for effective pollution management. Although the Phosphorus Index Model (PIM) has been widely applied and developed to evaluate phosphorus loss risks and identify key source areas in small watersheds, research on such risk assessments in wheat-corn rotation fields at different scales remains limited. In this study, a phosphorus index evaluation system was constructed using surplus phosphorus from a wheat-corn rotation system and available soil phosphorus as the source factor, and surface runoff, distance to rivers, and distance to lakes as the transport factors. The phosphorus loss risks and key nfluencing factors in wheat-corn rotation fields in different years (2015, 2017, and 2019) and scales (sources, transport to rivers, and transport to the lake) were examined, with a focus on the core areas of the Baiyangdian Basin, specifically Baoding, Dingzhou, and Xiong’an New Area. The results showed that during the study period, the key source areas and phosphorus losses were primarily concentrated in the northeastern and southwestern agricultural regions of the study area, with a 2.70% reduction in extremely high-risk zones area from 2015 to 2019. The key risk areas for phosphorus transport to rivers were more prevalent along both sides of the rivers, with a 0.95% decrease in extremely high-risk zones area during the same period. The critical source areas for phosphorus transport to the lakes were primarily located in the southwestern and northeastern counties surrounding the target water body, showing a 3.00% reduction in the extremely high-risk zones area. The key factors influencing phosphorus transport to the lakes included agricultural phosphorus surplus and transport factors (e.g., surface runoff and distance to the lake), both of which were significantly correlated with phosphorus loss risk (P < 0.01). Agricultural phosphorus surplus, primarily sourced from chemical fertilizers (average 69.32%), emerged as the main influencing factor, with an average correlation of 0.72 (P < 0.01). These findings provide a scientific reference for assessing phosphorus loss risks in the Baiyangdian Basin and offer guidance for the effective management of non-point source agricultural pollution.